Silver-containing conductive coatings are generally known in the art. Attention in this respect is directed to U.S. Pat. No. 4,369,063 to McGowan, Jr.; U.S. Pat. No. 4,029,605 to Kosiorek; and U.S. Pat. No. 4,975,301 to Andrews et al.
Such coatings are useful for various purposes such as heating elements, printed circuits and the like. They can be used as defrost elements for automobile rear windows, as heating elements for warming trays, as conductive elements for touch panels, in printed circuits for electronic applications, and for the manufacture of electronic components such as resistors and capacitors.
One of the principal uses for these coatings is as defrost circuits for automobile backlights (rear windows). Conventional silver-containing conductive coatings for such use generally consist of a particulate silver in a glass frit system. This conventional silver and glass system may contain various elements as modifiers to enhance properties such as bond strength, solderability and abrasion resistance. Sulfur-containing compounds, such as silver sulfate, copper sulfate, silver sulfate, etc., are frequently added in order to "stain" the glass substrate when the compositions are applied. Thus, they create a dark appearance when viewing the applied coating through the glass. These compositions are also dispersed in a vehicle suitable for the intended area of application. For example, pine oil-based vehicles are frequently utilized for screen printing applications.
The method of application of the compositions to the glass substrate is conventional and typically includes well-known techniques such as spraying, brushing, dipping, bonding or screen or stencil printing.
Following application of the composition to the substrate in the desired pattern or design, the coating is optionally dried and fired. The firing fuses the frit and effects a firm bond between the coating and the substrate.
When using such coatings in automotive backlights, after the conductive coating has been sufficiently fused to the glass substrate by means of the high temperature firing, electrical connectors, either tabs or wire mesh braids, are soldered to the surface of the conductive coating to complete the electrical connection to the power source. A problem is encountered in conventional conductive coatings in that solder wet through migration occurs. This causes an undesirable appearance of a "wet spot" where the solder was applied to the conductive coating. This "wet spot" can be seen when viewing the conductive coating from the opposite side of the glass substrate (viewing through the glass). The present invention provides conductive coatings which eliminate this solder wet through migration problem.
The silver-containing conductive coating compositions which are now practically used in the industry employ lead-containing glass frits. The lead content of these compositions creates certain environmental problems. The compositions of the present invention are lead-free and thus avoid these environmental problems while retaining or even exceeding the solder adhesion strength and abrasion resistance of these conventionally employed lead-containing coatings.
It is known that the content of glass frit in silver-containing conductive coatings has a profound effect on the desirable properties of such coatings such as adhesion to the substrate, solderability and resistivity. A low content of frit provides maximum solderability properties and as the content of frit increases, the solderability decreases. On the other hand, however, at low frit content the coating adhesion to the substrate is minimal and increases as the frit content increases. Resistivity of the coating is high at lower frit content and decreases to a minimal value as frit content increases. At a certain frit concentration, the resistivity reverses and begins to again increase as frit content increases.
For a given glass frit it is thus necessary to adjust the frit content of the composition so that the above properties are all maintained within acceptable values. In the case of conventionally employed lead-containing glass frits, it is necessary to adjust the frit content to relatively low values in order to maintain all the adhesion, solderability and resistivity values within acceptable parameters. In view of the differences in density between the lead-free glass frits employed in the present invention when compared to the lead-containing frits, it is, however, possible to increase the glass frit content in the compositions of the invention to higher amounts while maintaining acceptable values for all of these properties. This higher glass frit content is theorized to account for the stability to overcome the solder wet through migration problem. Further, the higher frit content is believed to achieve enhanced color characteristics for the fired conductive coatings and also to improve conductivity characteristics.